All 22 fighters who participated in The Ultimate Fighter season 16 Finale returned negative results from drug tests taken during the Dec. 15 event.
The results were announced by Nevada state athletic commission executive director Keith Kizer, who distributed them to MMA Fighting and other media outlets via email.
Fighters are tested for drugs of abuse as well as performance-enhancers.
The event took place in Las Vegas' Hard Rock Hotel & Casino.
Nelson also enrolled in and completed testing through the Voluntary Anti-Drug Association, an independent testing agency that conducts more rigorous drug screenings. He follows B.J. Penn and Rory MacDonald as MMA fighters who have done so.